The present invention relates generally to continuously variable transmission units (“variators”) of the toroidal-race rolling-traction type.
Major components of a known toroidal-race rolling-traction variator 10 of the “full toroidal” type are illustrated in FIG. 1. Here, two input discs 12,14 are mounted upon a drive shaft 16 for rotation therewith and have respective part toroidal surface 18, 20 facing toward corresponding part toroidal surfaces 22, 24 formed upon a central output disc 26. The output disc is journalled such as to be rotatable independently of the shaft 16. Drive from an engine or other prime mover, input via the shaft 16 and input discs 12, 14, is transferred to the Output disc 26 via a set of rollers disposed in the toroidal cavities. A single representative roller 28 is illustrated but typically three such rollers are provided in both cavities. An end load applied across the input discs 12, 14 by a hydraulic end loading device 15 provides contact forces between rollers and discs to enable the transfer of drive. Drive is taken from the output disc to further parts of the transmission, typically an epicyclic mixer, as is well known in the art and described eg. in European patent application 85308344.2 (published as EP 0185463). Each roller is mounted in a respective carriage 30 which is itself coupled to a hydraulic actuator 32 whereby a controlled translational force can be applied to the roller/carriage combination along a direction generally transverse to the main axis defined by the shaft 16.
The roller's motion can be broken down into three components:—
i. a rotary bearing 35 of the carriage 30 allows the roller to rotate about its own axis of symmetry (“the roller axis”) when driven by the associated input disc 12 or 14 and of course it is this rotary motion which transmits drive between the variator discs;
ii. in the prior art arrangement of FIG. 1 the piston 34 of the actuator 32 is capable of rotating within its cylinder, with consequent precession of the roller. That is, the carriage 30, piston 34 and roller 28 can rotate about an axis CA. The term “precession” in this context is used to refer to a rotation of the roller axis. Equivalently one may say that such precession involves a change in the inclination of the roller. The axis CA about which the roller precesses is referred to as the “castor axis”; and
iii. the roller/carriage can move translationally along a direction circumferential to the main axis as the piston 34 moves along its cylinder. In such translational motion the roller centre is constrained to follow the centre circle of the torus defined by the variator discs, since to depart from this circle would mean forcing the discs further apart against the end load.
As the skilled person is aware, the aforesaid precession of the roller about the castor axis changes the relative diameters of the paths traced out by a roller on its associated input and output discs, thereby changing the variator transmission ratio. It can be seen in FIG. 1 that the castor axis CA, determined by the positioning of the actuator 32 relative to the discs 12, 26, is at an angle C to a plane which is normal to the main axis. This angle C is the “castor angle” and has an important effect on variator control. When the variator is running, the effect of the discs upon the roller is to urge it toward an orientation in which the roller axis intersects the main axis, as is well known to those skilled in the art. Translational movement of the roller along the circumferential direction, eg. due to a change in force applied by the actuator 32, tends to move the roller axis away from intersection with the main axis. However such translational movement is accompanied by precession of the roller about the castor axis which, by virtue of the castor angle, brings the roller axis back into intersection with the main axis and hence to a stable position.
The FIG. 1 variator is of “torque control” type. The (controllable) force applied to each roller by its associated actuator 32 must for equilibrium be balanced by the reaction forces exerted on the roller by the adjacent discs. The net force exerted on the roller by the discs is proportional to the so-called reaction torque. Consequently the rollers automatically move and precess to positions in which they serve to transmit a torque determined by the actuator forces. The principle is well known in the art.
Some further variator design issues must be understood in order to appreciate the present invention in all its different embodiments.
The magnitude of the castor angle affects variator performance. A small castor angle can result in the variator being under damped, giving the rollers a tendency to overshoot the equilibrium position and oscillate undesirably. The geometry illustrated in FIG. 1 imposes a limit on the castor angle related to the diameter of the discs and their separation. While an appropriate castor angle is achieved in known variators of this type, greater design freedom would be available if this limit could be avoided.
The bulk of the variator and its exterior shape are highly important since it must typically be installed in the limited space available within a motor vehicle engine bay. Practical embodiments of the FIG. 1 arrangement have had “lobes” projecting at the exterior of the variator casing to accommodate the actuators 32 which, being of double acting type, (with working chambers on either side of the pistons) are unavoidably lengthy and which have had to be positioned radially outward of the discs. These lobes can create difficulties in fitting the variator into a vehicle. More generally, it has long been recognised that the variator design has a large volume of empty space within it which serves no useful purpose, in the toroidal cavities between the discs, and that it would be advantageous to put this space to use in order to reduce total variator volume: where the rollers are controlled by linear actuators such as hydraulic pistons, this has up to now proven to be challenging.
There have been variators in which the rollers were controlled by means of an opposed pair of single acting pistons, one at either end of the roller carriage, each received in a respective cylinder. The pistons and cylinders in such arrangements lay on a common axis (the castor axis) angled to the radial plane (the angle in question being the castor angle) to enable rotation of the roller. Here again, however, the actuators lay radially outward of the discs creating a bulky, irregularly shaped package.
A further design consideration is referred to herein as “axial compliance”. Under the considerable axial force applied by the end loading device 15, which is varied in operation in relation to the reaction torque, the toroidal faces 18, 20 of the discs move in a direction along the main axis, largely due to compliance in the discs, their mountings, the shaft etc. Such compliant movement of the discs can be of the order of 1 mm. The rollers 28 must be capable of some corresponding movement along the direction of the main axis along with the disc faces. This movement is straightforwardly provided in the FIG. 1 arrangement by slight angular movement of the piston 34 in its cylinder. Providing the required axial movement in the type of arrangement referred to above, in which each roller is arranged between a pair of actuators, is less straightforward.
A final design consideration concerns take-off of drive from the variator. This can be by means of a chain running on gear teeth formed on the radially outer surface of the central disc 26. However it is preferable in some contexts (particularly where the transmission is to be used in a rear or four wheel drive vehicle) to arrange for drive to be taken from the central disc to a component lying on the main axis. This type of “co-axial drive” can be very compact and it is desirable to provide for it in a simple manner.
In the course of a prior art review conducted prior to filing for a patent, the applicant has become aware of GB patent number 1002479, filed in 1964, published Aug. 25, 1965. Disclosed therein is a variator whose rollers are mounted to roller carrying “arms” in a manner which allows the roller axes to precess relative to the arms to alter the variator transmission ratio. However the variator in question is of a type in which all three rollers are to be actuated by means of a common central sleeve, rotation of which displaces the arms 42 and hence the rollers. Such arrangements proved impractical. To achieve “equalisation”—ie to ensure that all of the rollers, actuated by the common sleeve, arranged themselves to run at the same ratio required mechanical arrangement which proved unsatisfactory.